The present invention relates generally to stress testing timing and synchronization in data packet networks, and more particularly to connectionless configurations for stress testing.
An ongoing development in telecommunications is the convergence of voice, video, and data into a common stream. This requires migrating services typically delivered using a circuit network, such as telephony, to a packet based network. However, in a packet based network, synchronization of such services is difficult because there is no longer a precise network clock traceable signal as in a circuit switched network. The network traceable clock is used to recover the service clock of these circuit switched services (e.g., DS1, E1) to ensure error free-transmission. Circuit switched networks rely on the physical layer to transport these network clock signals between network elements to form a timing chain. The accuracy of these physical layer clock signals are typically synchronized to an accuracy of ±4.6 ppm or better. However, in packet networks, the clock signals used at the physical layer do not form a timing chain but are controlled by local free-running oscillators. Further, the accuracy of physical layer transport clock is synchronized to an accuracy of ±100 ppm Therefore, the physical layer clock signals in a packet network are not sufficient to support the error-free transport of circuit switched services over a packet network, commonly called circuit emulation. As a result, other methods must be used to recovery the service clock of circuit emulation services. The method of adaptive timing recovery typically relies on the arrival characteristics of packets as a basis to create a suitable service clock for circuit emulation.
ITU-T Recommendation G.8261—Timing and Synchronization in Packet Networks includes recommendations for testing methodology for testing circuit emulation and adaptive packet timing recovery. FIG. 1 illustrates conventional testing methodology described in G.8261 for testing adaptive packet timing recovery. As illustrated in FIG. 1, a packet-based equipment clock (PEC) server 102 generates a stream of packets referred to as the packet traffic of interest (PTI). An interworking function IWF of the PEC server 102 can generate the PTI by converting time division multiplex (TDM) traffic to packets. TDM traffic generally refers to asynchronous bit streams used in telephony networks (e.g., DS1, E1). The PTI is transmitted through Ethernet switches 104, 106, 108, and 110, to a device under test (DUT) 122, which is a PEC client. An IWF of the DUT 122 converts the PTI to a TDM signal to provide an emulated service. Background traffic generated by a traffic generators 120 and 114 are added to the PTI at each of the Ethernet switches 104, 106, 108, and 110 in order to generate delay variation (PDV) in the PTI. Testing equipment 116 is used to test the PDV generated in the PTI based on a reference timing signal 124, or primary reference clock (PRC), used to represent the TDM service clock. Testing equipment 118 is used to test jitter, wander (MTIE), phase deviation (TDEV, minTDEV), and frequency accuracy of the TDM signal output from the DUT 122.
As illustrated in FIG. 1, the current methodology described in G.8261 is based on connection-oriented topologies which ensure that the test traffic follows the same path, and therefore, is delivered in proper order. In addition, the delay distributions of such a configuration will tend to be statistically stable and allow timing recovery algorithms based on the central limit theorem to be used. Since these conditions do not reflect true network stresses or transient events, they will do little more than provide proof-of-concept of basic timing recovery operation. The focus of stress testing should be based on subjecting packet traffic of interest (PTI) to a series of controlled tests that impart a known stimulus for a specified duration with an expected result. The testing stimulus should be based on events that reflect stresses found in a real network. Stress conditions are not reflective of typical, or non-fault network conditions. Therefore, a testing methodology that supports both connectionless and connection-oriented networks is needed to provide a suitable stress testing environment.